The following abbreviations are herewith defined:    3GPP Third Generation Partnership Project    AT Allocation Table    DL Downlink (Node B to UE)    DRX Discontinuous Reception    GSM Global System for Mobile Communication    IMS Instant Messaging Service    LTE Long Term Evolution    Node B Base Station    RNC Radio Network Control    RRC Radio Resource Control    SIB System Information Block    UE User Equipment    UL Uplink (UE to Node B)    UMTS Universal Mobile Telecommunications System    UTRAN UMTS Terrestrial Radio Access Network    E-UTRAN Evolved UTRAN
In 3.9G (E-UTRAN) it has been agreed that the mobility principle for a UE in the Active state will be network controlled handover. This means that when the UE is in the Active state the UE is expected to provide the network with measurement reports for providing the network with information to assist it in making a HO decision. UE performed measurements, which form the basis of the measurement reports send by the UE to the network, are normally performed when the UE is active for transmission and/or reception of data. A current general view in 3.9G is that data transmission and/or reception is linked to the DRX scheme of the UE. The DRX scheme basically pertains to when the UE wakes up from an inactive period (sleep) in order to receive information as to whether any UL or DL resources have been assigned for the UE. In 3.9G the resource assignment is proposed to be done through ATs.
It has been realized that combining Active state mobility through network controlled HO may not be fully compatible with another 3.9G requirement concerning optimized power saving possibilities for those UEs in the Active state. Enhanced power saving possibilities may have the side effect of introducing long delays in the measurements performed by the UE, and thus in the measurements reports sent to the network from the UE. This delay from the UE side in providing the measurement reports to the network can be expected to introduce an overall delay in the HO decision made by the network, thereby delaying the overall HO procedure and adversely affecting the mobility of the UE. In a worst case the delay in making a HO decision in the network may cause increased inter-cell interference and possibly a loss of service.
The GSM system uses HO when in the dedicated mode (circuit switched mode), otherwise cell reselection is performed as either network assisted or stand-alone. The UTRAN system supports both HO and cell reselection as well.
In a 3GPP 3.9G study item the problem stated above been has been acknowledged, and various proposals have been made.
A first proposal would keep the UE in the Active state only for very short periods of time (10-20 Seconds) and let the transition to the idle state be done without signaling between the network and the UE (e.g. timer based). However, a problem with this proposal is the handling of error cases when the UE and the network are unsynchronized concerning the UE state (Active or Idle), which has the potential to introduce unacceptable behavior.
Another proposal also would keep the UE in the Active state only for very short periods of time (10-20 Seconds), but would then let the transition be accomplished using, e.g., RRC signaling between network and UE. A potential problem with this proposal is that a UE which performs frequent IP transfers (e.g., e-mail, TCP/IP keep alive, IMS keep alive) may cause excessive signaling to be performed due to the frequent RRC state transfers.
Another proposal introduces pull-mode/re-active HO in the Active state (which resembles cell reselection).
Prior to this invention, no completely satisfactory solution has been proposed to the problems discussed above.